In many installations particularly of an industrial nature there are produced gases that are hot and that contain moisture. Various means have been proposed in the past for recovering the heat from these waste gas streams and certain of these prior devices utilize the waste heat to heat a space such as a room or the interior of a building. In certain of these recuperator structures, especially where ambient air is used for the air supply to the space, problems are created in cold climates in cold seasons of the year with a frost being deposited on the heat exchangers used in the recuperator structures. The apparatus of this invention provides improved means for recovering the heat values from a moisture laden hot gas stream and for using ambient air to absorb the heat and be heated thereby for supply to the space. The present invention also provides means for periodically defrosting the recuperator system so that the frost will not interfere with the gas flow through the structure of this invention.